This invention relates to heat-insulating paper containers, and more particularly to heat-insulating paper cups used for coffee and other hot or cold beverages.
Several types of heat-insulating cups have been used commercially for hot and cold liquids. Examples include the wide variety of existing polystyrene foam cups. These cups are typically made by adding a foaming agent to a polystyrene resin, casting the mixture into a mold, heating the resin under pressure to foam it, and removing the shaped article from the mold. Such cups have outstanding heat-insulating properties, but require a lot of energy to make, and are costly to dispose of in an environmentally acceptable way.
A variety of paper-based cups have been proposed as environmentally acceptable alternatives to polystyrene containers. Unfortunately, most attempts to produce paper-based heat-insulating cups have proved costly due to the complexity of their manufacturing processes. One example is a cup where the side wall is surrounded by a corrugated heat-insulating jacket. Its process for manufacture involves additional steps of forming the corrugated paper jacket and bonding it to the outer surface of the side wall of the cup. These cups, however, have proved to be aesthetically unappealing and structurally deficient. In this regard, only the ridges of the corrugated jacket contact the body of the paper cup in such a way that the bond between the two is so weak that the cup and jacket easily separate. Also such cups are not easily nested making storage difficult.
Another type of insulating paper cup is that where the cup has a dual cup structure. An inner cup is given a different taper than an outer cup to form a heat-insulting air layer in between the two cups. The two cups are integrated by curling their respective upper edges into a brim. The dual structure adds to manufacturing costs, and the two cups of the structure are prone to separation.
One of the most widely accepted types of heat-insulating paper-based cups include those described in U.S. Pat. No. 4,435,344, and also referred to in U.S. Pat. No. 5,490,631. Those cups have good insulting properties and can be prepared at a relatively low cost. Such cups are fabricated from a body member and a bottom member, both cut from a paper sheet.
For the cups described in the aforementioned U.S. patents, one surface of the body member is coated or laminated with a thermoplastic synthetic resin film, and the other surface of the body member is coated or laminated with the same or different thermoplastic synthetic resin film or an aluminum foil, to thereby foam the thermoplastic synthetic resin film and form a heat-insulting layer on at least one surface of the container, i.e., the outer surface. Water present in the paper is vaporized upon heating during processing, causing the thermoplastic resin film on the outer surface to foam.
Commercial versions of cups prepared according to U.S. Pat. No. 4,435,344 include a body member and a bottom panel member. The body member comprises a paper sheet coated or laminated on one side with a foamed heat-insulating layer of low density polyethylene which entirely covers its outer surface. The inner surface of the body member is covered with an unfoamed film of high density polyethylene. The bottom panel member is laminated on its inner or upper surface with low density polyethylene film. The inside surface of the body member, and the inside surface of the bottom member are coated to prevent penetration of liquid contents into the paper sheet during use. The inside surface of the body member is also coated to ensure that the water in the paper will not evaporate directly into the air atmosphere during heating of the fabricated cup.
For these commercial cups, the film to be laminated on the inner surface of the body member is high density polyethylene because it has a higher softening point than the low density polyethylene film on the outer surface. Upon heating during fabrication, the moisture inherently present in paper sheet serves as the foaming agent for the outer surface layer of low density polyethylene film. The high density polyethylene on the inside surface of the body member will not foam under fabrication conditions but serves to seal the interior of the cup thus preventing an escape of the moisture necessary to foam the low density polyethylene on the outer surface of the cup.
A significant drawback of the aforementioned cups is that they are difficult to adequately seal when fabricated under desirable commercial conditions, i.e., high cup-forming machine speeds. When assembling such cups using conventional cup-forming machines, the bottom panel member and the body member are oriented in such a way that upon folding, heating and sealing the body member inner layer of high density polyethylene and the unfoamed polyethylene inner layer of the bottom panel member are in contact. Applicants have found that when fabricated under high cup-forming machine speeds leaking occurs at this juncture between inner layer of high density polyethylene of the body member, and the unfoamed polyethylene inner layer of the bottom panel member.
One object of the invention, therefore, was to develop a heat-insulating paper cup which does not leak when fabricated under high cup-forming machine speeds. While numerous exotic solutions could be envisioned involving a redesign of the cup structure and/or cup-forming machines, it was a further object of the invention to develop such a cup which could be fabricated still using conventional and existing cup-forming machines. In this regard, it was also an object of the invention to address the leaking problem while maintaining as many of the conventional operating conditions and materials as possible thereby avoiding extensive redesigns of existing mass production operations for commercial cups.